Airborne geophysical measurements depend on the use of a stable platform that is precisely located geographically, able to incorporate sensors aligned in three orthogonal directions, and able to follow the topography closely, without any disturbance, especially from the towing craft.
Therefore, the platform needs to be situated at a sufficient distance from the towing craft so as to be free of its field of influence. It needs to be stable, in the sense that any pitch, roll or yaw movements are minimized so that the wanted signal is not affected significantly. Such measurements are most commonly taken using fixed-wing aircraft, but these craft do not fully satisfy the criteria of separation from the platform, or that of being able to closely follow the topography. The helicopter, on the other hand, is able to fully satisfy these two criteria.
Most geophysical methods require differential measurements, in other words, they record components of the signal through the use of suitably distanced sensors capable of measuring the rate of change of the signal. When this rate of change is measured in three orthogonal directions, these measurements can be used to calculate the resultant vector, called the total gradient. In general, this value contains more information than the actual field value, mostly because it allows one to remove uncertainties due to approximations, for example those due to the distance of the base station from the sensors in the case of magnetic measurements.
Magnetic gradiometer measurements are one of the possible applications of the platform presented here. It is the method most suitable for describing the principals used, and is hence used to illustrate the advantages of the present platform.
The first trials to produce such a system began in the 1960's for magnetic field measurements. References are herein provided to map some history of airborne magnetic gradiometry measuring systems.
Papers by Hood, P. J., 1965 (Gradient Measurements in Aeromagnetic Surveying; Geophysics, Vol. 30, p. 891-902) and Hood, P. J. and Teskey, D. J., 1989 (Aeromagnetic Gradiometer Program of the Geological Society of Canada, Geophysics, Vol. 54, p. 1012-1022), describe existing systems through a retrospective examination. Papers by Hood, P. J. and Teskey, D. J. 1987 (Helicopter-borne Aeromagnetic Gradiometer Surveys: A Progress Report, in Current Research, Part A. Geol. Surv. Can., Paper 87-1A, p. 935-938), show the advantages of differential measurements for mineral prospecting.
Thus, in 1984 at least four systems were known to exist for this activity. All of them were built based on the same model, as presented in FIG. 2 of the paper by Hood, P. J. and Teskey, D. J., 1987 (Helicopter-borne Aeromagnetic Gradiometer Surveys: A Progress Report, in Current Research, Part A. Geol. Surv. Can., Paper 87-1A, p. 935-938), that of a “bird” at the end of a tow-cable (towed bird), above which a vertical mast around 2 meters in length is usually attached.
To summarize, the principal characteristics of this model comprise one main longitudinal axis; a secondary vertical axis placed above the main axis, and a pulling point located above the group formed by these two axes.
The various models proposed subsequently are in fact simply variants of the model described above, since they all use a main longitudinal axis—the bird—above or below which a vertical axis is attached. The two most commonly described models were initially proposed by Gamey, J. T., Holladay, J. S. and Mahler, R., 1997 (Airborne Measured Analytic Signal for UXO Detection, Environmental and Engineering Geophysical Society (EEGS), SAGEEP proceedings, from 853 and onwards), and by Berger, Z., Davies, J., Thompson, R. T., McConnell, T. J., Lo B., Ryder-Turner, A. and MacKay, P., 1999 (Exploration Applications of Three Dimensional Gradient Magnetics in the Western Canada Sedimentary Basin and the Fold Best Region, Reservoir, September 1999), and finally by Siegel, H. O., McConnell, T. J. and Ryder-Turner, A., 2001 (Method and Apparatus for Detecting Locating and Resolving Buried Pipelines, Cased Wells and Other Ferrous Objects, U.S. Pat. No. 6,255,825 B1, Date of Patent Jul. 3, 2001).
The last publication above claims both a methodology and a system for the detection and positioning of ferrous magnetic objects. The claimed methodology is based on the use of the “analytical signal”, as is commonly known through the work of Nabighian, published in 1972 (Nabighian, M. N., 1972, The Analytic Signal of Two-dimensional Magnetic Bodies With Polygonal Cross-Section: Its Properties and Use for Automated Anomaly Interpretation: Geophysics, Vol. 37 (3), p. 507-517) and more recently by Roest, W. R., Verhoef, J., and Pilkington, M., in 1992 (Roest, W. R., Verhoef, J., Pilkington, M., 1992, Magnetic Interpretation Using the 3-D Analytic Signal: Geophysics, Vol. 57 (1), p. 116-125). Its application for ferrous magnetic object detection has been previously presented by Gamey, J. T., Holladay, J. S. and Mahler, R. in 1997 (Airborne Measured Analytic Signal for UXO Detection, Environmental and Engineering Geophysical Society (EEGS)., SAGEEP proceedings, from 853 and onwards).
The system described in the paper by Siegel, H. O., McConnell, T. J. and Ryder-Turner, A., 2001 makes use of the three main characteristics as originally published by Hood, P. J. and Teskey, D. J., 1987 (Helicopter-borne Aeromagnetic Gradiometer surveys: A Progress Report, in Current Research, Part A, Geol. Surv. Can., Paper 87-1A, p. 935-938).